The top-drive assembly in land-based and offshore drilling rigs provides the rotational force needed to drill a borehole. Typically, several cables supply power to the motors within the top-drive assembly.
In conventional designs, these power cables are positioned within large-diameter rubber hoses with each power cable typically being secured to a rubber hose with a flexible epoxy. Each rubber hose may be clamped to a steel cable, which provides support to the power cables. The rubber hoses protect the power cables from harsh conditions experienced during drilling operations. Indeed, rubber hoses are used to protect the power cables, because conventional cable jackets do not provide sufficient mechanical protection.
The rubber hoses (and the power cables) typically are suspended from a position about midway on the drilling rig in a service loop. The service loop provides cable slack, thereby allowing the top-drive assembly to vertically reciprocate (i.e., move up and down the drilling rig).
Because each power cable in conventional designs is secured within a rubber hose (e.g., with an epoxy) that vertically reciprocates corresponding with the movement of the top-drive assembly, it is important for the power cable to be designed for continuous flexing operations. A cable having insufficient flexibility (e.g., not designed for continuous flexing) may suffer from undesirable fatigue and eventually break.
Furthermore, problems may arise if each power cable is not centered within its rubber hose in the service loop. A power cable that is not centered will have a different loop radius than the rubber hose. Whenever the power cable bends during operation (e.g., caused by the vertical reciprocation of the top-drive assembly), stresses occur in the power cable. Thus, if a power cable is not centered within the rubber hose, it will experience non-uniform stress, which can lead to the premature failure of the cable.
Another problem of conventional designs is that the power cable may become twisted because of the continuous reciprocation of the top-drive assembly.
The conductors within the power cable can also cause undesirable twisting. In addition to the hose, conductors within the power cable partially support the weight of the power cable. These elements, however, elongate at different rates, causing the conductors to become the primary support mechanism of the power cable. This, in turn, can lead to the power cable becoming twisted. Power cables employing a single conductor are particularly susceptible to such twisting. This twisting causes additional stresses in the power cable and eventually premature failure.
Accordingly, a need exists for an improved top-drive power cable that (i) resists cable rotation and (ii) does not need to be positioned within a rubber hose.